A method for regenerating an exhaust aftertreatment system is carried out as a function of defined engine operating states based on parameters which are relevant in this respect. The regeneration cycles include the actual regeneration operation and a specific period between the individual regeneration operations. The regeneration in part requires changes in the engine operating state in the particular instantaneous engine operating point characterized by load torque and rotational speed. These may be, for example, measures to increase the exhaust gas temperature during the thermal diesel particulate filter regeneration. Measures to increase the exhaust gas temperature are, for example, retarding the start of injection of the main injection or an additional fuel injection in the same working cycle after the main injection as a postinjection. Under unfavorable operating conditions, increased exhaust gas emissions may occur during the regeneration.
After the regeneration is initiated, depending on the design of the exhaust aftertreatment system, specific engine operating points should no longer be started during the course of the regeneration. For example, during the thermal regeneration of diesel particulate filters, a transition into overrun condition or idling after the start of regeneration is detrimental to the completeness of the soot burn-off or will even damage the system if the filter is too heavily loaded.
After the characteristic ignition temperature is exceeded in the diesel particulate filter, an exothermic process accompanied by a temperature increase occurs in the filter. The magnitude of the temperature increase may depend, among other things, on the composition and quantity of the reaction gas or the exhaust gas (oxygen concentration, exhaust gas mass flow, the particulate mass in the filter and the exhaust gas temperature upstream of the filter. When a transition is made into overrun operation or idling, there is the danger that if the filter is heavily loaded, the exhaust gas mass flow diminishes, while the partial pressure of oxygen simultaneously increases. This may result in a temperature increase in the diesel particulate filter, which may cause its destruction. Other relationships apply, for example, to NOx desorption exhaust aftertreatment systems in accumulator-type catalytic converters.
The particulate filter systems developed in recent years make it possible to strongly reduce particle emissions in diesel vehicles. The reduction in emissions amounts, for example, to more than 97% of the particulate mass. At certain time intervals, it is necessary to empty the particulate filter of its soot deposits so that the flow resistance does not reduce engine performance. To that end, the soot layer is burned off, forming carbon dioxide (CO2) and water vapor. Exhaust gas temperatures exceeding, for example, 550° C. are required for burning the soot. Such temperatures are not reliably reached in vehicle operation, making additional measures necessary for the regeneration. Basically, it is possible to distinguish between active and passive systems for the regeneration. Even when using systems through which the soot ignition temperature is reduced, such as a catalytic soot filter, a CRT system or catalytic fuel additives, active measures must be employed to be able to implement reliable filter operation.
The CRT system functions as soon as exhaust gas temperatures greater than 250° C. are reached. However, it is not possible to always ensure this in modern diesel vehicles, making it possible for excess particulate matter to accumulate in the filter, resulting in high backpressures and a reduction of performance in the internal combustion engines. In such cases, normally active systems must be switched on in order to reach the necessary exhaust gas temperatures. However, all such measures require additional energy (heat) which is ultimately generated from the fuel (e.g., electrical energy from the battery to generate heat in a heating element, a fuel burner, postinjection of fuel into the engine combustion chamber). The exhaust emission control system thus increases the fuel consumption of the vehicle. Even in other exhaust emission control systems or exhaust aftertreatment systems such as the accumulator-type NOx catalytic converter, specific temperature ranges are required in order to be able to ensure the function (regeneration). The regeneration measures, i.e., among other things, the selection of the point in time at which a regeneration of a particulate filter is initiated, are presently oriented, for example, to the distance traveled (e.g., after 400 to 700 km) and the pressure differential across the particulate filter. As soon as the pressure differential has increased by a specific value or a specific distance has been traveled, the regeneration is initiated if specific preconditions have been met which make a regeneration possible, such as engine temperature, exhaust gas temperature or the like. In general, in previous exhaust aftertreatment systems, a regeneration is implemented as a function of the load condition and the operating state of the internal combustion engine.
With consideration of the relationships explained above, an object of the present invention is to provide a method for regenerating an exhaust aftertreatment system via which the danger of harmful effects on the exhaust aftertreatment system and the internal combustion engine is minimized while maintaining optimum exhaust gas values.